1. Field of the Invention
The present invention relates to the preparation of monohalohydroquinones, and especially to the preparation of monochlorohydroquinone by halogenation of hydroquinone.
2. Description of the Prior Art
Monohalohydroquinones, and in particular monochloroand monobromohydroquinones, are important industrial compounds useful as intermediates in organic synthesis, in the photographic industry (see U.S. Pat. Nos. 2,748,173 and 1,912,744) or as dihydroxylated compounds for the preparation of anisotropic polyesters (see U.S. Pat. No. 4,118,372 and French patent application No. 79/24,135, published under No. 2,465,758). Monochlorohydroquinone, in particular, is in marked demand for these uses.
Although various syntheses for the preparation of the monohalohydroquinones, and in particular of monochlorohydroquinone, have been proposed to this art, starting with compounds such as p-benzoquinone and ortho-chlorophenol, the most advantageous method from an industrial standpoint consists in the halogenation of hydroquinone, a common and inexpensive material. To this end, several processes have been described. Thus, in U.S. Pat. No. 1,912,744 it was recommended to prepare monochloro- and monobromohydroquinones by passing chlorine or bromine into a suspension of hydroquinone in carbon tetrachloride. According to U.S. Pat. No. 2,748,173, monochlorohydroquinone is prepared by passing chlorine gas into a concentrated solution of hydroquinone in aqueous acetic acid. These two processes have the major disadvantage of leading, whatever precautions are taken, to the concomitant formation of 2,3- and/or 2,5-dichlorohydroquinones in amounts which become greater as the degree of conversion of the hydroquinone increases.
To remedy this disadvantage, it was proposed, in published Japanese Application No. 56/45,433, to carry out the chlorination of hydroquinone using dilute hydrochloric acid in the presence of copper salts or iron salts as catalysts, and working under an oxygen pressure. Although this process appears to be conducive to more selective production of monochlorohydroquinone, its value is reduced by the need to resort to pressure-resistant equipment and to the use of metal salts. In U.S. Pat. Nos. 4,439,595 and 4,439,596 it was proposed to replace chlorine by sulfuryl chloride and to perform the chlorination in alkyl esters or in glacial acetic acid in order to obtain mixtures having a high monochlorohydroquinone content, the mixtures being directly usable for the preparation of the corresponding diacetate intended for the production of anisotropic polyesters. In this case, the presence of significant amounts of dichlorohydroquinones and of unconverted hydroquinone is not prejudicial to the desired object; the yields of dichlorohydroquinone relative to the hydroquinone converted remain greater than 10% and the degree of conversion of hydroquinone less than 90%.
It has also been proposed to conduct the halogenation of hydroquinone using halogens in the nascent state, formed "in situ" by oxidation of a hydracid with an oxidizing agent. Thus, in British Pat. No. 563,541, a process has been described for preparing mono- or polychlorohydroquinones via the nascent chlorine formed by adding manganese dioxide to a suspension of hydroquinone in concentrated hydrochloric acid. The use of the manganese oxide limits the industrial value of this process because it involves the handling and addition of a solid material to the suspension of hydroquinone in hydrochloric acid, and requires a laborious regeneration of the manganese oxide from the manganese chloride by-product. In French patent application No. 73/20,507, published under No. 2,187,735, it has been proposed to conduct the halogenation of various aromatic compounds, including hydroquinone, using a halogen (chlorine, bromine, iodine)/hydrogen peroxide system in order to recover, in the form of halogen, the hydracid released by the direct halogenation of the aromatic compound. This process hence employs a first ordinary stage of halogenation of the substrate with a deficit of halogen relative to stoichiometry, and a second stage of halogenation of the unconverted substrate using the halogen formed "in situ" by oxidation of the hydracid, which is a by-product, with hydrogen peroxide. This process is advantageously carried out in a two-phase system containing an aqueous phase and an organic phase. It has been found that, in the case of the chlorination of hydroquinone, this process does not remedy the disadvantages of the earlier processes, particularly as regards the formation of polychlorohydroquinones; furthermore, a large amount of hydroquinone undergoes, under these conditions, oxygenation by the hydrogen peroxide and/or the nascent chlorine, especially when the medium is heterogeneous.
Thus, despite the extensive research heretofore conducted in this art, serious need continues to exist for a process by which hydroquinone can be halogenated simply, more selectively and more completely. Cf. Chemical Abstracts, 95, No. 9, p. 738, No. 80474 g (1981); U.S. Pat. No. 3,929,907; French Pat. No. 950,265.